Basically its about proper computation of flowrate, headloss and size of total filter. In my filter setup, the total area of each modules is 4.5 meters x .65meters x .9 meters for a total of 2.63 tons for an empty chamber. Once I put all the matts and brushes, I will get just 1.841 tons or 1840 liters of water(2.63 x 70%) inside this filter more or less.

My sequence pump is rated at 5800 gph but after considering the headloss in the piping and pump up to bakki it goes down to around 5000gph more or less or just 19,000 liters per hours. This translates to 316 liters per minute of pump action. 1840 liters divided by 315 liters per minutes would give around 5.8 minutes. I can confirm this shutting down the incoming bottom drain pipe and letting the external pump go on and then time how many minutes it takes for the pump to draw out half of the water in my whole filter. Normally it takes around less than three minutes to drain half of the filter module using the pump thereby confirming my computation.

If one would like to increase the retention time, you can either increase the filter space or reduce the flow rate of the pump using a valve or make a direction return pipe after the mechanical stage to return back to the pond thereby allowing faster mechanical filter turnover but longer bio filter retention time.

So after 4 years of having this pond, I finally did 2 things. One is to build a bigger pond and second is to renovate the older pond in the last 3 months.

The newer pond is around 36 tons inclusive of filter. The design concept is still the same as my older pond. It has 2 bottom drains and each bottom drains leads to separate "ERIC style horizontal flow filter with 2 sets of "Ebrushes and 4 sets of " EMatts" on each line. Each line is pump thru by a sequence pump at 5800gallons per hour. The 2 pumps deliver water to a 6meter concrete 2 layer shower with sintered glass medias. Two 15 watts UV is installed. Cleaning takes from 2 mins(for brushes and flush) to 15 minutes to clean the whole two lines. Flowrate is around 50 minutes. It now houses 11 gosanke that range from 70cm to 85cm that were previously house in the old 20 ton pond.

The old pond was also renovated. In four years, I lost only 1 koi(grown to 84cm) due to swim bladder complications. Never had problems with any pumps or controls. The brushes and matts still looked brand new. Pond never leaked as well. I was able to grow several koi to 80 to 85cm gosanke without any body or skin deterioration.

Among the renovation were the bakki shower was relocated with new tiles inplaced. The filter covers were replaced. The pond repainted. Filter was slightly sloped to improve flush. The matts were reduced. More green to make it softer to the eye. All previous multiple airpumps were removed. All aeration going to the pond and filters is pipe thru just one 0.5hp roots blower which powers also all the aeration of the bigger pond. Japanese airhose to the pond were replaced and pvc pipe with small holes now aerate the filter. Turnover remains the same at 30mins. Backup airpumps installed. As of the moment, no koi are house yet. I plan to house 7 or 9 koi in this pond.